Thanks for bringing this topic up. I am witnessing a number of my elder relatives going through dementia as we speak and it is really terrifying. Majority of them are over 70 and suffer from short-term memory loss. Commonly, they misplace things and end up accusing innocent people of stealing their belongings.
Of the 4 cases I know of, 2 of them weren't the most pleasant people pre-dementia and from my knowledge, is going through some tough times with their dementia e.g. little support from family. One elderly lady passed away a couple of years ago as a result of post-dementia parkinson and she spent some 10 years in a catatonic state before her heart stopped. The other secured her lonliness through her unresonable behaviour including accussing her care-taker of trying to seduce her husband (who by the way is no longer around).
The other 2 are older folks who in my opinion were generous and lived a morale life and is now being cared very well for by their family including one who has 3 children who are all medical doctors.
None however, are buddhist nor medidated in their lives. Having said that, I do know of one buddhist meditator who is currently going through a second childhood. But he seems to practise some form of off-brand meditation, dabbling in spirits, ghosts and what not. I am not trying to draw any conclusion. I am just sharing with you guys what I know base on my personal experience.
I did ask this question to a meditation teacher before and his response was that as it is right now, even without dementia, our minds are deluded by experiences generated through our sense bases. A well trained mind (will have a higher capacity to overcome delusions and dementia is just a disease that results in highten delusions. So I am guessing that as you progress in your practise and you happen to draw the demantia karma card somewhere down the line, one may be incapable of certain physical, bodily or mental functions but you will be able to keep calm regardless of whatever activity or inactivity that your sense bases is generating.
Having said that, the mere thought of going through some form of mental disease or dementia still terrifies me from time to time. This and given my family history, is why I am putting in extra effort in my training.